A flat-panel display is an image-producing device whose average lateral dimension along the display's viewing surface is considerably greater than the display's maximum thickness. A flat-panel display has an active image-producing portion having front and back surfaces which normally extend roughly parallel to each other and which are roughly flat. Examples of flat-panel displays are flat CRT displays, flat liquid-crystal displays (“LCDs”), flat electroluminescent displays, flat plasma displays, and flat light-emitting diode displays.
FIG. 1 schematically illustrates the active image-producing portion of a conventional flat-panel CRT display. The display contains backplate structure 20 and faceplate structure 22 connected together to form sealed enclosure 24. Backplate structure 20 contains multiple electron-emissive regions 26. Faceplate structure 22 contains multiple light-emissive elements 28 situated on transparent faceplate 30 respectively across from electron-emissive regions 26. Electrons emitted by regions 26 cause elements 28 to emit light that produces an image in an active display area at the exterior surface of faceplate 30.
The electrons emitted by each region 26 are intended to strike a corresponding target light-emissive element 28. However, some of the electrons emitted by each region 26 invariably strike the display outside target element 28. To prevent many of the off-target electrons from striking other elements 28 and causing them to emit light that degrades the image, elements 28 are laterally surrounded by a border region 32 which is substantially non-emissive of light when struck by these electrons.
Border region 32 normally contains black material which absorbs a large fraction of the ambient (external) light that impinges on region 32 from outside the display. As a result, “black matrix” 32 enhances the contrast between (a) the image light provided by light-emissive elements 28 and (b) the ambient light which strikes elements 28 and black matrix 32 in the display's active area. Unfortunately elements 28 typically reflect a substantial fraction of the ambient light that strikes them. As a result, the enhanced image contrast produced by black matrix 32 is still sometimes insufficient to achieve desired image clarity, particularly when the ambient light is of high magnitude.
Referring to FIG. 2, Hunt, U.S. Pat. No. 4,231,068, and Koehler/Beran et al, “A Unique Active Contrast-Enhancement Filter Using Liquid-Crystal Pi-Cell Technology,” SID 86 Digest, 1986, pages 436–438, discuss techniques for enhancing the contrast of an image produced by raster scanning in a conventional deflected-beam CRT display 40. The techniques of Hunt and Koehler/Beran et al entail placing multiple strips 42 of an LCD in front of display 40. Signals from a control box 44 are provided on electrical lines 46 to switch each of strips 42 between a light-transmissive state and a light-absorptive state. The switching is controlled so that each strip 42 (a) transmits light when writing occurs behind that strip 42 and (b) absorbs light when no writing occurs behind that strip 42. The image contrast is enhanced, especially in high ambient lighting conditions.
The contrast-enhancement techniques disclosed in Hunt and Koehler/Beran et al are creative. However, the LCD employed in Hunt to form strips 40 contains slow-switching twisted-nematic liquid crystal. It is doubtful that the switching speed obtainable with Hunt's display would be great enough for many future applications. Furthermore, the LCDs employed in Hunt and Koehler/Beran et al utilize polarizers which significantly reduce image intensity. The polarizers may be damaged by heat and/or humidity, and thus raise reliability concerns. Also, Hunt and Koehler/Beran et al are directed to raster-scanned CRT displays of the deflected-beam type. It is desirable to have a simple, reliable, fast-switching mechanism for enhancing the contrast in a display, especially a flat-panel display such as a CRT flat-panel display.